A capacitance type dynamical quantity sensor according to a prior art is disclosed in U.S. Pat. No. 6,151,966. FIG. 7 shows the dynamical quantity sensor 111 according to the prior art. The sensor 111 is an acceleration sensor, and has a movable electrode 16 and a pair of fixed electrodes 17, 18. The movable and fixed electrodes 16-18 face each other so that they form a capacitor. When acceleration is applied to the sensor in a direction Z, a movable portion 12 having the movable electrode 16 moves to some extent. Then, capacitance of the capacitor between the movable and fixed electrodes 16-18 is changed. This capacitance change is measured with an outer electric circuit so that the acceleration is detected.
FIG. 8 shows a schematic graph of a relationship between the capacitance change ΔC and vibration frequency ω of the movable portion 12. Here, the capacitance change ΔC represents a sensitivity of the sensor 111, and has the maximum value at resonant frequency ωn (n=1, 2, 3, . . . ). In other words, the capacitance change ΔC is peaked at the resonant frequency ωn.
When the acceleration is measured with the capacitance type dynamical quantity sensor 111, it is preferred that frequency dependence of the sensor sensitivity is small. That is because the measured value of the capacitance change is easily compensated by the outer electric circuit in case of small frequency dependence. Accordingly, when the resonant frequency ω1 of the movable portion 12 is disposed in a measurement frequency range Y, a spring constant K of the movable portion 12 is set to be larger, or mass M of the movable portion 12 is set to be smaller. Thus, the resonant frequency ωn of the movable portion 12 is changed from ω1 to ω2. Therefore, the resonant frequency ωn, i.e., ω2 becomes larger than the measurement frequency range Y. Thus, in the measurement frequency range Y, the frequency dependence of sensor sensitivity becomes small so that the measured value is easily compensated. However, the sensitivity itself becomes lower in the measurement frequency range Y, as shown by the solid line in FIG. 8.